Devices for suspending parts and controlling or damping their movement relative to one another are known in the art. For example, such devices are known and used in the automotive field in vehicle suspension systems. The devices might take the form of shock absorbers, struts, and other motion or vibration damping structures.
Generally, many of those devices utilize fluids for controlling the relative movement of the mechanical parts. For example, hydraulic fluid may be utilized as a medium for creating damping forces or torques or controlling motion. One class of such movement control devices utilizes a fluid medium that is controllable through the use of magnetic fields. Such magnetically controlled fluid is referred to as magnetorheological, or MR fluid. MR fluids exhibit a thickening behavior (a rheology change) upon being exposed to magnetic fields of sufficient strength. The higher the magnetic field strength to which the MR fluid is exposed, the higher the flow restriction or damping force that can be achieved in the MR device. It may be desirable to utilize this behavior of MR fluids to control the damping of an MR device, for example to modify the ride characteristics of a vehicle in response to driver input or road conditions.
An MR device may use an electromagnet comprising an electric current flowing through a coil to apply a magnetic field to affect the flow properties of the MR fluid. The damping force of the MR device is a function of the current supplied to the electromagnet. If no current is applied to the electromagnet, damping is at a minimum level. The damping level with no magnetic field applied may be lower than would be desirable for most vehicle operating conditions. For the majority of vehicle operating time, a current must be constantly supplied to provide a desired level of damping. This current requirement adds a load to the vehicle electrical system, which may adversely affect fuel economy. Also, the wire used to form the electromagnet, as well as the vehicle wiring harness and connectors, must be sized appropriately to accommodate the current level required. Additionally, the low damping level with no magnetic field applied may be undesirable from a vehicle control standpoint if there is an interruption of the control current to the damper.
In order to reduce operating current and/or to provide a desirable level of damping in the absence of current, it is known in the art to include a permanent magnet in an MR device to supply a bias magnetic flux. For example, U.S. Pat. Nos. 5,632,361 and 6,419,057 show such an approach. The bias flux increases the damping force of the MR device in the absence of current through the electromagnet. The electromagnet can be used to provide magnetic flux to supplement the bias flux.
In using an MR device, an important characteristic is what is referred to as the “turn-up ratio”. Turn-up ratio is defined as the ratio of the maximum force or torque generated by the MR device divided by the minimum force or torque output for the same device. In designing controllable MR actuators it is generally desirable to maximize the turn-up ratio under given operating conditions. The turn-up ratio is maximized by increasing the torque or force available when the MR fluid is exposed to a maximum magnetic field and/or by minimizing the torque or force output when the fluid is exposed to a minimum magnetic field. In a conventional MR device where the entire magnetic field is generated by an electromagnet, the minimum magnetic field is achieved by applying zero current to the electromagnet, which results in no induced magnetic field. For such a device turn-up ratio is dependent mainly on the characteristics of the MR fluid, namely the yield strength of the fluid with a magnetic field applied and the viscosity of the fluid in the absence of a magnetic field.
Adding a permanent magnet to an MR device to supply a bias magnetic flux can have undesirable effects. Prior art configurations are known that make it impossible to completely counter the bias flux, resulting in a significant level of flux always operating on the MR fluid to raise the minimum force output and thus lower the turn-up ratio. Known prior art configurations may require significant current levels to counter the bias flux to achieve low minimum force, as is required to achieve a desirably high turn-up ratio. Prior art configurations are also known that require expensive permanent magnet material to reduce the risk of the electromagnet's flux demagnetizing the permanent magnet.
Thus, what is desired is an MR damper that will reduce the current requirements in controlled operation and provide a greater than minimum level of damping when the power source is off, while maintaining a high turn-up ratio.